Recently digital broadcasting has started. In the digital TV set used for the digital broadcasting, graphical images are generated according to data from a broadcasting station and the generated graphical images are overlaid on video images of a program when displayed.
For this purpose, the digital TV set is equipped with an image compositing apparatus.
The image compositing apparatus performs a predetermined operation between pixels for images F and G to output an image H. In the inter-pixel operation, a pixel f(x,y) of the image F and a pixel g(x,y), which corresponds to the pixel f(x,y), of the image G are used to generate a pixel h(x,y) of the image H.
One method of the inter-pixel operation is the α blending algorithm. In the α blending algorithm, the weighted average of the pixels f(x,y) and g(x,y) is the pixel h(x,y). Suppose that the weighting factor for the pixel g(x,y) is “α”, the α blending algorithm is given below.h(x,y)=α*g(x,y)+(1−α)*f(x,y)
where, “*” indicates multiplication
Equation Group 1
In this equation, the weighting factor is referred to as an “αvalue”, a “transparency”, a “blending factor” or the like. The value of the weighting factor is “0” to “1”.
When the α value is “0”, the pixel g(x,y) is completely transparent and the pixel f(x, y) as it is becomes the combined pixel h(x,y). On the other hand, when the α value is “1”, the pixel g(x,y) is completely opaque and the pixel g(x,y) as it is becomes the pixel h(x,y). When the a value is “0.5”, the pixel h(x,y) is a composite of 50% of the pixel g(x,y) and 50% of the pixel f(x,y). In this manner, the α value indicates how an image is overlaid on another image. In the actual hardware, each pixel is represented by RGB (Red, Green, and Blue) color components, so that the arithmetic operation indicated above is performed for each of the components.
For compositing graphical images and video images together in real time, the image compositing apparatus includes an OSD (On Screen Display) plane, which is a memory area for developing graphical images, and a video plane, which is another memory area for developing video images in frame units. Whenever a frame on the video plane is updated, images in the two areas are composited together according to the α blending algorithm to output a composite image. FIG. 1A is a conceptual diagram showing how images on an OSD plane 2501 and a video plane 2502 are composited together to output a composite image 2503.
Meanwhile, the recent digital broadcasting requires to overlay a plurality of graphical images 2511, 2513, 2514, and 2515, which indicate the title and TV programs and the like, on a video image 2512 of a program while the video image 2512 is reproduced as shown in FIG. 1B.
Theoretically, it is enough to perform the a blending algorithm for images to be composited together in turn from the bottom to the top in order to composite a video image and a plurality of graphical images together. Actually, however, a large amount of arithmetic operations is performed for the α blending algorithm for a plurality of images, so that it is extremely difficult to composite a plurality of images in real time whenever a frame of video images is updated.
Images can be composited at high speed by providing the planes that correspond to the images and a plurality of pieces of hardware that perform the a blending algorithm and that also correspond to the images. In this case, however, hardware cost is raised. Also, many planes are needed. This is problematic.